Aquaculture, also known as the farming of aquatic organisms, provides nearly one third of all seafood consumed in the world at present. It constitutes an alternative to fishing and is expected to expand with the increase of consumers' demand for seafood and the decrease in wild supplies (Timmons, Ebeling, Wheaton, Summerfelt, and Vinci, 2002, Recirculating Aquaculture Systems, 2nd Edition, 760, Cayuga Aqua Ventures, Ithaca, N.Y.).
Aquaculture systems can be extensive, semi-extensive, or intensive, depending on the number of organisms grown per volume of water. An exemplary extensive aquaculture system is a pond culture. Cage culture is considered semi-intensive outside the cage and intensive inside the cage. The main disadvantage associated with these aquaculture systems is their high water consumption (typically 3-5 m3 per kg fish produced). These systems also tend to be environmentally unfriendly.
Recirculated Aquaculture Systems (RAS) are intensive aquaculture systems which were developed and refined over the past thirty years. These systems provide a controlled environment in which fish grow. Consequently, fish can be stocked at high to very high densities, depending on the fish species. In these systems fish are raised in tanks, sometimes within closed buildings, while water is recycled throughout the system and various treatment units enable maintenance of adequate water quality. In this manner only a small percentage of the water is exchanged daily. Failure of any one of the treatment units can cause the entire system to fail, usually killing the fish population. RAS have relatively low water consumption and thus can be built at favorable locations, with less dependency on the water source. RAS further provide year-round production, mitigation of environmental risks (e.g. diseases) and pollution. RAS are species-adaptable, allowing operators to follow market trends for seafood preference. Moreover, RAS constitute a “point source” of pollution, which enable efficient solids waste treatment and nutrient removal and are thus considered environmentally friendly.
The high fish densities in RAS require efficient gas-transfer systems which dissolve oxygen and remove carbon dioxide from the culture water. Additionally, nonionized ammonia (NH3) is toxic to many fish species at concentrations as low as 0.0125 mg NH3—N L−1. In order to avoid the accumulation of ammonia, a nitrification unit is employed to reduce the total ammonia nitrogen (TAN) to concentrations typically below 3 mg L−1 (warm-water fish; Timmons et al., 2002). One equivalent of alkalinity is generated for each equivalent of ammonia (NH3) excreted by the fish gills when converted to the ammonium ion (NH4+) at pH close to neutral, at which the pond is typically operated. Nitrification consumes approximately two equivalents of alkalinity per mole of oxidized ammonium. Thus, in RAS with a nitrification unit, only one equivalent of alkalinity is lost per mole nitrogen excreted by the fish. In high-density cultures, this alkalinity loss can lead to the elimination of the water buffer capacity, which may result in a pH drop, which in turn hinders nitrification, causes ammonia to accumulate, and may finally result in fish death. In order to avoid this scenario, a strong base such as sodium hydroxide (NaOH) or a weak base such as sodium bicarbonate (NaHCO3) is usually added to the system. Alternatively, the make-up water flow rate can be increased or the effluents of a denitrification reactor can be recycled back into the pond.
Treatment of aquaculture farm effluents can amount to 2%-10% of the total production costs (Cripps and Bergheim, 2000, Aquacultural Engineering, 22, 33-56) which is a major disadvantage as compared to the less sophisticated aquaculture facilities, such as earthen ponds and fish cages. Profitability of recirculating systems depends in part on the ability to manage nutrient wastes (Van Rijn, Tal and Schreier, 2006, Aquacultural Engineering 34, 364-376).
Currently, most RAS configurations do not include a nitrate removal unit, and hence nitrate concentration in the effluents is set only by the make-up water exchange rate. Intensive nitrate removal is feasible only if a denitrification reactor is employed (e.g. in zero discharge systems). Typically, only ammonia (which is immediately toxic to the fish) is removed, while nitrate and dissolved phosphorus species (which constitute an environmental problem but are not considered toxic at reasonable concentrations) accumulate in the water and are disposed of with the discharge from the pond. The removal of all the nutrients is employed in several RAS technologies by applying a closed loop nitrification-denitrification sequence. The removal of phosphorous species is usually performed by the addition of chemicals. The conventional technique for N species removal consists of biological ammonia removal by nitrifying bacteria. However, nitrifying bacteria are autotrophic organisms with a long doubling time and low biomass yield. Therefore, these systems suffer from long start-up periods and when failure occurs, the recovery of the bacterial population is slow. Moreover, when a denitrification system is employed and its effluent is recycled back into the system, turbidity may develop in the pond which increases the potential for the outbreak of disease. The reduction of make-up water and the recycle of the water through the denitrification reactor have also been associated with the development of off-flavor in the fish.
US 2003/0052062 discloses a nitrogen treating method wherein a nitrogen compound in for-treatment water is treated according to an electrochemical technique with hypohalogenous acid, or ozone or active oxygen.
US 2009/0317308 discloses catalysts for converting ammonia in an aqueous solution directly to nitrogen gas at about or above ambient temperature.
US 2009/0317308 further provides a method for water treatment to lower its ammonia content by converting the ammonia to nitrogen directly in aqueous phase.
U.S. Pat. No. 4,522,727 discloses a continuous process for removal of ammoniacal nitrogen from water with a particulate zeolitic ion exchange material that is continuously regenerated by heating in the presence of an oxygen-containing gas.
U.S. Pat. No. 5,512,182 discloses a method for removing trace amounts of ammonia and ammonia-containing compounds from process water involving the destruction of the ammonia by oxidative procedures.
US 2004/0134796 discloses an apparatus for diminishing the concentration of ammonium in wastewater, and for disposing of the ammonium as nitrogen gas, the apparatus includes an ammonium-extraction-and-transfer station which is operable to transfer the ammonium extracted from the stream of wastewater into solution, and an electrolysis station to electrolyze the secondary-water and to oxidize the ammonium dissolved therein to nitrogen gas. The apparatus further includes a nitrogen-discharge port, for discharging the resulting nitrogen gas from the electrolysis station.
Seed et al. (www.enpar-tech.com/documents/AmmEL_WEAO_2003.pdf) discloses an ion-exchange/electrochemical technology to treat ammonia in wastewater streams which uses electrochemistry to convert ammonia to nitrogen gas.
There is an unmet need for economical and reliable technique for removal of nitrogen and phosphorus species which are excreted by the fish in a recirculated aquaculture system without the use of hazardous chemicals.